Editing VM (operating system) (section)

== Shared File System ==
[[File:VM370 Rel 6 COBOL compile.png|thumb|Invoking the System/360 COBOL compiler on VM/370 CMS, then loading and running the program]]
With modern VM versions, most of the system can be installed to SFS, with the few remaining minidisks being the ones absolutely necessary for the system to start up, and the ones being owned by the filepool server machines.
[[File:VM370 Rel 6 starting up DOSVS Rel 34.png|thumb|An example of a non-CMS guest operating system running under VM/370: DOS/VS Release 34. The DOS/VS system is now prompting the operator to enter a supervisor name to continue loading]]

VM/SP Release 6 introduced the Shared File System <ref>{{cite manual|url=http://bitsavers.org/pdf/ibm/370/VM_SP/Release_6_Jul88/SC19-6210-05_VM_SP_Release_6_CMS_Users_Guide_Jul1988.pdf|title=VIrtual Machine/System Product CMS User's Guide Release 6|at=Chapter 4 (Using the Shared File System)|date=July 1988|id=SC19-6210-05|publisher=[[IBM]]|access-date=August 19, 2021|archive-date=June 17, 2022|archive-url=https://web.archive.org/web/20220617123632/http://bitsavers.org/pdf/ibm/370/VM_SP/Release_6_Jul88/SC19-6210-05_VM_SP_Release_6_CMS_Users_Guide_Jul1988.pdf|url-status=live}}</ref> which vastly improved CMS file storage capabilities. The CMS minidisk file system does not support directories (folders) at all, however, the SFS does. SFS also introduces more granular security. With CMS minidisks, the system can be configured to allow or deny users read-only or read-write access to a disk, but single files cannot have the same security. SFS alleviates this, and vastly improves performance.

The SFS is provided by service virtual machines. On a modern VM system, there are usually three that are required: VMSERVR, the "recovery machine" that does not actually serve any files; VMSERVS, the server for the VMSYS filepool; and VMSERVU, the server for the VMSYSU (user) filepool.<ref>{{cite manual | title = CMS File Pool Planning, Administration, and Operation | id = SC24-6261-02 | date = November 12, 2021 | url = https://www.vm.ibm.com/library/720pdfs/72626102.pdf | section = File Pool Server Machines | section-url = https://www.vm.ibm.com/library/720pdfs/72626102.pdf#page=18 | pages = 18–23 | series = z/VM 7.2 | access-date = June 10, 2022 | publisher = [[IBM]] | archive-date = October 6, 2022 | archive-url = https://web.archive.org/web/20221006041027/http://www.vm.ibm.com/library/720pdfs/72626102.pdf | url-status = live }}</ref> The file pool server machines own several minidisks, usually including a CMS A-disk (virtual device address 191, containing the file pool configuration files), a control disk, a log disk, and any number of data disks that actually store user files.

If a user account is configured to only use SFS (and does not own any minidisks), the user's A-disk will be <code>FILEPOOL:USERID.</code> and any subsequent directories that the user creates will be <code>FILEPOOL:USERID.DIR1.DIR2.DIR3</code> where the equivalent UNIX file path is <code>/dir1/dir2/dir3</code>. SFS directories can have much more granular access controls when compared to minidisks (which, as mentioned above, can often only have a read password, a write password, and a multi-write password). SFS directories also solve the issues that may arise when two users write to the same CMS minidisk at the same time, which may cause disk corruption (as the CMS VM performing the writes may be unaware that another CMS instance is also writing to the minidisk). 

The file pool server machines also serve a closely related filesystem: the Byte File System. BFS is used to store files on a UNIX-style filesystem. Its primary use is for the VM OpenExtensions POSIX environment for CMS. The CMS user virtual machines themselves communicate with the SFS server virtual machines through the IUCV mechanism.<ref>{{Cite web |title=IUCV Overview |url=https://www.ibm.com/docs/en/zvm/7.1?topic=vehicle-iucv-overview |access-date=2022-07-31 |website=www.ibm.com |language=en-us |archive-date=July 31, 2022 |archive-url=https://web.archive.org/web/20220731215201/https://www.ibm.com/docs/en/zvm/7.1?topic=vehicle-iucv-overview |url-status=live }}</ref>